1. Field of the Invention
The present invention relates to capacitors, and more particularly, the present invention relates to high farad, double-layer capacitors that include improved electrodes which are enclosed within a sealed plastic case.
2. Description of the Prior Art
Conventional double-layer capacitors include two polarizable bodies which are made of a paste of activated carbon and sulfuric acid. The individual carbon paste bodies are held apart, but disposed in close proximity by a porous separator. Electrical contact with each of the carbon paste bodies is established by means of conductive electrodes. Double-layer capacitors store energy by forming a polarized liquid layer at the surface of the conductive electrodes.
A typical double-layer capacitor contains a single cell. Such a capacitor includes a pair of current collectors or current electrodes; a pair of polarized electrodes separated by a non-woven fabric or porous separator; and a gasket positioned between the current electrodes and surrounding the polarized electrodes and the separator. The polarized electrodes of such a capacitor are manufactured from activated carbon and are impregnated with an electrolyte such as, for example, an acid.
One of the shortcomings inherent with a polarized capacitor of the construction discussed above is its internal electrical resistance. In most prior-art capacitors, the individual particles of carbon in the polarized electrode are not joined together. This physical environment causes the internal resistance of the electrode to be high. In order to reduce the internal resistance of the polarized electrode it is necessary to bring all the particles of carbon into improved electrical contact with each other.
A further shortcoming with the prior-art construction of polarized capacitors is that the internal resistance of conventional double-layer capacitors is also greatly affected by the contact resistance between the collector electrodes and the polarized electrodes. To reduce the contact resistance between the polarized electrodes and the collector electrodes, and to further reduce the internal resistance of the polarized electrodes, the capacitor cells of the prior art are kept under pressure. Such pressure normally brings the particles of activated carbon into improved electrical contact with each other. Additionally, the pressure also brings the polarized and collector electrodes into improved electrical contact with each other. In this regard, conventional double-layer capacitors are normally kept under a pressure of about 100 kg/cm.sup.2. Prior-art, double-layer capacitors are kept under pressure by deforming their outer cases, which are usually made from metal. Another method used to apply the appropriate pressure involves bonding the collector electrodes strongly to the gaskets.
The capacitance of a double-layer capacitor may be improved by increasing the cross-sectional area of the basic cell. However, when the cross-sectional area of the cell is increased, the pressure applied to the double-layer capacitor must correspondingly increase. Increasing the pressure causes practical problems such as finding a means for applying the pressure, and increasing the strength of the outer case which encloses the basic cell.
In addition to the problems noted above, double-layer capacitors suffer from additional problems. One such problem is leaking. Double-layer capacitors typically use an acid as an electrolyte, and because the casing which encloses the capacitor is typically metal, the capacitors corrode and subsequently leak. In addition to the identified leaking, the gaskets employed in conventional double-layer capacitors are subject to degradation, which manifests itself by cracking and wrinkling, and which is a result of corrosion, as well as heat induced expansion and retraction. Accordingly, conventional capacitors may experience leaking in the area around and through their respective gaskets.
The prior art is replete with numerous examples of assorted devices and assemblies which have attempted to improve conventional double-layer capacitor design. Many of these prior-art attempts have been directed to increasing the capacitance of a double-layer capacitor without increasing the external pressure applied to the basic cell. For example, U.S. Pat. No. 5,086,373 ("the '373 patent"), which issued to Kurabayashi, discloses a double-layer capacitor which has a construction where the carbon particles in the polarized electrode are joined together by sintering the electrode. In such a sintered electrode arrangement, the individual particles of carbon are joined together, but the polarized electrode remains relatively porous. Accordingly, the internal resistance of the electrode is reduced, while the surface area of the respective electrodes remains relatively high. To further improve the electrical contact between the polarized electrodes and the collector electrodes, the device described in the '373 patent employs individual collector electrodes which are manufactured by a technique which includes hot curing a mixture of rubber and conductive particles and applying it to the polarized electrodes. The hot curing process results in the rubber flowing into the porous polarized electrode and, thereby, increasing the area of contact between the polarized electrodes and the collector electrodes.
While the process disclosed in the '373 patent results in an improved capacitor which eliminates some of the shortcomings attributed to conventional designs, it is still unsatisfactory for applications where a capacitor having a very large capacitance is desirable. Such applications include so-called "SLI" or starting, lighting, and ignition applications. Typically, in such applications a relatively large capacitance capacitor is electrically coupled to a battery, or other voltage source, which is used to start a machine such as an industrial engine or motor. In these circumstances, the starting of the engine requires a significant amount of current early in the starting process, and then less current as the starting proceeds. Capacitors are used to provide the initial current in such an application, thereby reducing the current demands on the battery coupled to it. The battery provides the rest of the current necessary to start the engine.
In addition to their unsatisfactory operation in SLI applications, conventional double-layer capacitors, are generally expensive, heavy, and difficult to dispose of or recycle once they have reached the end of their commercial usefulness.
As noted, the capacitance of a capacitor may be increased by increasing the area of the basic cell. As described, most double-layer capacitors utilize a paste consisting of carbon powder and an electrolyte. Another factor upon which the capacitance of double-layer capacitors depends is the active surface area of the carbon powder used in the paste. If an improved powder could be developed for use in such pastes, the performance of capacitors could also be improved.
Accordingly, it would be desirable to have a double-layer capacitor having even greater capacitance than prior-art devices. It would also be desirable to have a capacitor which is relatively lightweight and relatively inexpensive. It would also be desirable to have a capacitor which has an advantageous combination of capacitance, weight, and cost characteristics, unachieved heretofore, and to have a capacitor which is substantially leak-free and which can be enclosed within a corrosion-resistant container. Finally, it would be desirable to have a capacitor which is manufactured from materials which can be readily recycled.